Aluminum alloy 6020 was developed in 1992 for cold finished product possessing good machinability. Cold finished products include wire, rod, and bar applications that have been used in the automotive and commercial industries. Machinability can be defined as the relative ease with which the material can be machined. Machining processes include such processes as roughing, finishing, and milling. Good machinability is difficult to measure, however one ranking system that has been used for some time classifies machinability based on a letter scale with an “A” rating being most machinable, followed by “B”, “C”, “D” and “E” ratings taking into account the following characteristics:
(1) Chip Size. Smaller chip sizes are more desired because such chips simplify the machining operation and facilitate more effective heat removal from the tool work piece interface than larger chips. Chips must not be too small or they interfere with lubricant recirculation during the overall machining operation, such as by drilling or cutting. Long, thin chips by contrast tend to curl around themselves rather than break. Such chips, sometimes called curlings, may require manual removal from the machining area and are less effective than smaller chips at heat dissipation because larger chips tend to block the cooling lubricant.
(2) Tool Wear. Lower tool wear rates are desired to save money by increasing the amount of time a tool can be used before prescribed tolerances for a given work piece are exceeded. Lower tool wear rates further increase productivity by reducing downtime due to tool changeovers.
(3) Surface Finish. Alloys exhibiting a very smooth exterior surface finish in the as-machined condition are more desired to eliminate or reduce the need for subsequent surface finishing operations, such as grinding and deburring.
(4) Machining Forces. Lower machining forces are more desired to: reduce power requirements and the amount of frictional heat generated in the work piece, tool and tool head; or increase the amount of machining or metal removal that can be accomplished with the same power requirements; and
(5) Mechanical and Corrosion Properties. Mechanical characteristics such as strength, or other properties such as corrosion resistance, may be “optional” with respect to machinability. They can also be rather important depending on the intended end use for the work piece being machined.
Although this “A” through “E” rating system is based on the five parameters discussed above, the relative importance of each parameter changes as a function of intended end use for any given alloy.
The desire to get lead out of the alloy for environmental reasons drove the development of aluminum alloy 6020. It was desired to extend this alloy to a press quenched product to also address environmental issues related to press quench aluminum alloy 6262-T6. A press quenched product is one that has been rapidly cooled from an elevated deformation extrusion temperature by immersion in a liquid bath, such as oil or water, so as to withdraw heat rapidly from the product. Air can also be used as a substitute for liquid. The purpose of quenching is to suppress a phase transformation so as to obtain increased hardness, or other desirable properties. The severity of the quench depends upon the capacity of the liquid or air to withdraw heat rapidly from the metal, this in turn depending upon other factors, such as the latent heat of vaporization, thermal conductivity, specific heat, and viscosity of the liquid or air.
Attempts to extend 6020 to a press quenched product were met with several problems. One problem was that magnesium (Mg) combined with tin (Sn) during billet reheat, which resulted in low strength, such as tensile strength, and poor machinability. Tensile strength is the resistance of a product to a force tending to tear it apart, measured as the maximum tension the product can withstand without tearing. When an aluminum alloy product, such as a billet or ingot, is extruded, it is first reheated to and held at a temperature in the alloy above the solubility temperature in the precipitated phases in the aluminum matrix, for instance the solubility temperature for the magnesium (Mg)-silicon (Si) phases in a billet made of an Al—Mg—Si-alloy, until the phases are dissolved. The product is then quickly cooled or quenched to the desired extrusion temperature to prevent new precipitation of these phases in the alloy structure. Between the temperatures of 800° F. and 920° F., magnesium combines with tin at a rapid rate to form magnesium tin. Above 920° F., the magnesium and tin do not combine and will actually dissociate from each other. Below 800° F., the reaction is sluggish and there is typically not enough time during billet reheat for these two elements to substantially combine. The product forms for which a press quenched 6020 alloy is desired are rod, bar, and wire applications. For press quench products of this nature, billet temperatures of 825 to 900° F. are typically utilized. As described above, this temperature range will not allow alloy 6020 to achieve acceptable machinability in the press quenched product.
In addition, other problems encountered were that there was a low producability compared to 6262 when overcoming this magnesium-tin combination issue and there was a lack of an optimized composition within the sales limits. In extrusion, the higher the billet temperature, the slower the extrusion speed that can be attained. As described, previously the preferred billet temperature range for alloys such as 6262 is 800 to 920° F. As aforementioned, these temperatures resulted in unacceptable machinability for the 6020 alloy. Going to higher billet temperatures resulted in a significant loss of extrusion productivity. Additionally, the composition was not optimized for press quenched products. It was discovered that higher magnesium levels resulted in a greater degradation to machinability. The higher Mg levels provide a higher driving force to promote the formation of Mg2Sn below approximately 920° F. To counter this effect, the magnesium level is optimized towards the lower side of the sales limits. Additionally, the tin level was maximized to maintain a higher volume fraction of the desirable Sn phase that provides the favorable machining characteristics of 6020. However, with lower magnesium levels, the strength in the final product is compromised. To offset this Si levels are optimized towards the higher side of sales limits.
The primary object of the present invention is to provide a substantially lead free alloy that is press quenchable.
Another object of the present invention is to provide a press quench alloy with enhanced extrusion productivity and good mechanical properties and machinability.
A further object of the invention is to provide a press quench alloy that can be used as a direct replacement for lead containing alloy 6262-T6.